Tunable 24-GHz Antenna Arrays Based on Nanocrystalline Graphite

Abstract

In this work, we present a tunable 24-GHz antenna array based on a CMOS-compatible 110nm-thick nanocrystalline graphite film grown by plasma enhanced chemical vapor deposition. The film has a nominal bulk conductivity exceeding 16000 S/m (hence, greater than any graphene monolayer or industrially available graphene multilayer) but still able to show an outstanding modulation of its charge carrier density in the upper microwave spectrum. The manufactured layer was used to design, simulate, fabricate, and test a 24-GHz patch antenna array, with each radiating element having overall dimensions of just λ0/8×λ0/7. The fabricated array exhibits a measured maximum gain of about 3 dBi around 24 GHz (unbiased state), with a half-power beam width of only 14.5° (suitable in wireless links where a high directivity is envisaged). Spanning the dc bias voltage between -25 V and 25 V at 24 GHz, the gain can be tuned continuously between -1.5 dBi and 4 dBi, whereas the resonance frequency undergoes a maximum shift of 166 MHz. This voltage-dependent tuning of the gain represents a first step in developing carbonbased applications to control amplitude and phase simultaneously and independently. These results (never reported) demonstrate the big potential of nanocrystalline graphite for high-performance microwave components that are CMOS compatible (a highly desirable characteristic for high fabrication yield and largescale production), with unprecedented tunability for next-generation high-capacity communications.

Field-Effect Transistors Based on Single-Layer Graphene and Graphene-Derived Materials

“…The deposition technique offers the advantage of growing the films directly on an insulating layer (i.e., SiO ), thus eliminating the need for an ulterior transfer step, which, for example, is required for the SLG thin films. The bulk-NCG thin films are grown with a previously established process [ 37 , 38 , 39 ], in a CH :H (60:75 sccm) atmosphere, at a pressure of 200 Pa and an RF discharge power of 100 W. The substrate temperature is kept at a high value of 890–900 °C, over a two h plasma growth step, to promote a high nucleation density. This in turn will result in thin films with high edge defect density, which is beneficial for sensors that rely on conductivity variations due to particle surface adsorption [ 37 ].…”

“…This in turn will result in thin films with high edge defect density, which is beneficial for sensors that rely on conductivity variations due to particle surface adsorption [ 37 ]. The PECVD process yields bulk-NCG thin films of ≊360–380 nm thickness with a conductivity of over S m [ 38 , 39 ]. For the GNW thin films, the discharge power is increased to 300 W and the process pressure is lowered to 40 Pa to stimulate the growth-induced vertical morphology.…”

Mechanically Reconfigurable Waveguide Filter Based on Glide Symmetry at Millimetre-Wave Bands

“…In this field, CMOS-based devices [ 22 , 23 , 24 , 25 , 26 , 27 ] are also widely employed because of their high level of integration, mass production cost, and combination of RF devices, digital and analogue circuits. However, devices in this technology still consume a lot of power for millimetre-wave band applications.…”

Evolution of Nanocrystalline Graphite’s Physical Properties during Film Formation

“…Another notable process parameter that may not seem highly impactful at first is the plasma growth time, with which the film thickness can be controlled. Although film thickness is the direct consequence of varying the exposure time to the plasma phase, other intrinsic properties also suffer significant changes (e.g., electrical conductivity [6,11]).…”

“…Moreover, it is arguably the easiest one to modify and is readily available to do so on any deposition system. Although studies that include time-dependent deposition properties have been presented in the past, they usually focus more deeply on other process parameters, and the scope of said papers is not solely aimed at the investigation of growth time dependence [6,7,11]. For example, Schmidt et al [7] presented a comprehensive study of NCG thin films' properties with respect to several growth parameters, including deposition time, but the paper concentrates on a larger array of parameters and analyses a growth period of only 15 min.…”